1. Field of the Invention
The present invention relates to a negative resist composition, in particular, a chemically amplified negative resist composition; and to a patterning process that uses the negative resist composition.
2. Description of the Related Art
As packing density and speed of LSIs have become higher, a finer pattern rule has been increasingly realized. In 1994, volume production of 180 nm rule devices was scheduled to start in 2001 on the SIA road map. Actually, the volume production has been pushed forward by 2 years and begun in 1999. Although ArF (193 nm) lithography was seen as a promising technique for production of 180 nm devices, KrF (248 nm) lithography has been continuously used. Then as to the 150 nm generation, even as to 130 nm generation, volume production by KrF lithography has been considered.
As KrF lithography is reaching maturity, finer dimensions have been increasingly realized. It is expected that use of ArF realizes microprocessing in 90 nm, and use of F2 (157 nm) realizes microprocessing in 65 nm. Beyond this, techniques suggested as being promising are EB reduction projection exposure (PREVAIL SCALPEL) or EUV using soft X-ray as a light source.
Conventionally, polymers for resists were changed significantly every time a wavelength of light was shifted. This was done in order to secure transmittance needed. For example, in the shift from g-line to i-line, a base of sensitizers was changed from benzophenone to non-benzophenone types. The shift from i-line to KrF involved a change from novolac resins, which were used for a long period, to hydroxy styrenes. In the shift from KrF to ArF, the change of polymers is drastic and alicyclic polymers are going to be used because light cannot pass through polymers having a double bond at all. Furthermore, in F2, in order to enhance transmittance further, use of alicyclic polymers to which fluorine atoms are introduced such as fluororesins has been investigated.
In using a high energy beam having an extremely short wavelength such as EB or X-rays, light elements used in resists such as hydrocarbons hardly absorb the high energy beam. Therefore, polyhydroxystyrene based resist compositions have been investigated.
Resists for EB have actually been used for mask lithography. In recent years, techniques for fabricating masks have been perceived as problems. Since the era of using g-line, reduction projection exposure systems have been used and its reduction ratio has been 1/5, and 1/4 reduction ratio has begun to be used recently along with enlargement of chip sizes and use of projection lenses having larger apertures. Not only reduction of line width due to realization of finer processings but also reduction of line width due to the change of reduction ratio are huge problems to techniques for fabricating masks.
As for exposure systems for fabricating masks, in order to enhance precision of line width, use of exposure systems using electron beams (EB) have been started instead of exposure systems using laser beams. Furthermore, still finer dimensions are achieved by increasing acceleration voltage of EB in an electron gun. Therefore, the acceleration voltage is sifted from 10 keV to 30 keV, and then use of 50 keV has been going mainstream recently.
Then as the acceleration voltage increases, a problem occurs that sensitivities of resists are deteriorated. When the acceleration voltage increases, forward scattering has less influence in resist films, the contrast of electron lithography energy is enhanced, and thus resolution and dimensional controllability are enhanced. However, electrons pass through resist films without being hampered. As a result, sensitivity of the resist is deteriorated. With a mask exposure system, exposure is conducted by direct and one time writing. Therefore, deterioration of resist sensitivity leads to decrease of productivity, and which is not preferable. To meet the demand of high sensitivity, a chemically amplified resist composition have been examined.
Accordingly, the increase of acceleration voltage and application of a chemically amplified resist exhibiting high contrast make it possible to write a dimension of 500 nm, which is 125 nm on a wafer by 1/4 reduction. However, use of KrF lithography has been extended up to device dimension of 130 nm, ArF lithography is said to be applied from 90 nm, and F2 lithography is estimated to be applied from 65 nm. The limit of optical lithography using F2 is estimated to be 50 nm. At this time, the dimension on a mask is 200 nm. At present, line width control of 200 nm is difficult to achieve by enhancing resolution of resists. In the case of optical lithography, use of thinner resists considerably contributes to enhancement of resolution. This is because introduction of CMP and so on have resulted in an advanced stage of planarization of devices. In the case of fabricating masks, substrates are flat, and thicknesses of substrates to be processed (for example, Cr, MoSi, or SiO2) are determined for imperviousness to light and control of phase contrast. In order to use thinner resists, there is no other choice but to enhance dry etching resistance of resists.
It should be noted that it is generally believed that there is a correlation between carbon density and dry etching resistance of a resist. For EB lithography, which is not influenced by absorption, novolac polymer based resists having excellent etching resistance have been developed. However, it is difficult to control molecular weights and polydispersities of novolac polymers, and thus novolac polymers are not suitable for microprocessings.
In addition, it is reported that absorption of carbon atoms is small in soft X ray (EUV) exposure at a wavelength of 5 to 20 nm which is expected along with F2 exposure as an exposure technique for microprocessings of 70 nm or beyond. It has been found that increase of carbon density not only enhances dry etching resistance but also effectively enhances transmittance in soft X ray wavelength region (see N. Matsuzawa et. al.; Jp. J. Appl. Phys. Vol. 38 p 7109-7113 (1999)).
As mentioned above, a resist composition having high carbon density, high dry etching resistance and high resolution has been demanded.